This invention relates to electrodes and has particular reference to anodes for use in diaphragm or membrane cells.
It is well known to produce chlorine by the electrolysis of alkaline metal chlorides, particularly sodium chloride in diaphragm type electrolytic cells. Basically these cells comprise alternate anodes and cathodes which are separated by a diaphragm typically formed of asbestos. The diaphragm serves to separate the anolyte and the catholyte liquors. A more recent development is the membrane cell in which the diaphragm is replaced by a membrane permeable to one ion species only.
Initially the anodes for diaphragm cells were formed of graphite. More recently permanent metal anodes have been introduced, these normally being formed of titanium and having an anodically active surface layer on the titanium sheets. A typical diaphragm anode comprises an elongate current feeder--normally a copper cored titanium bar--to which is spot-welded two titanium sheets of open mesh construction. The two sheets are arranged parallel to one another on opposite sides of the current feeder. Around part of the periphery the two sheets are joined together to form an open box type structure.
A more recent development of a diaphragm cell anode is described in British Pat. No. 1326673 (and in its equivalent U.S. Pat. No. 3,674,676). This patent describes the adjustable diaphragm cell anode structure in which the connection between the feeder and the anode working faces or sheets is movable so that the distance between the two opposed sheets can be varied whilst retaining the electrical integrity of the assembly. The adjustability is most clearly shown in FIG. 6 of the British patent.
In practice, however, it has been found that a number of problems are associated with adjustable anodes of this type. Because the anodes are quite tall--usually about 2 ft tall--and because the distance between the faces is quite small--usually about 1-2 in--it is difficult to adjust the thickness of the anode and at the same time to make effective mechanical electrical connections within the anode once it is installed in the diaphragm cell. It is not normally practical to adjust the anode to its final size before insertion into the cell because of the dangers of damaging the diaphragm. A further problem is that once adjusted the anode is of fixed dimensions and any deterioration in the diaphragm can cause the diaphragm to be moved away from the anode and thereby increase the electrical resistivity of the cell.
In an attempt to overcome the problems associated with the arrangement illustrated in FIG. 6 alternative forms of construction are illustrated in FIGS. 4 and 5 of the aforementioned British patent specification. With such an arrangement, however, it is necessary to insert spacer bars such as bars 17 (FIGS. 3 and 5 of the aforementioned British patent specification) to space out the anode working faces. Because of the constrictions within the diaphragm or membrane cell this is an added complication on assembly.
An alternative proposal in the aforementioned British patent specification is illustrated in FIGS. 8 and 9. In this arrangement the anode working surfaces are connected to the central current feeder by means of spring members. The anode can be assembled outside the cell with the working surfaces contracted and on insertion the clips holding the working surfaces together are removed permitting the anode working faces to spring outwardly. Because of the movement on expansion it is necessary to provide a gap between halves of the working surface and the gap is shown clearly by reference numeral 39 in FIG. 8. This gap does, however, form a potential source of damage to the diaphragm or membrane since it might pinch the diaphragm or membrane and possibly pierce it. Also the construction illustrated in FIGS. 8 and 9 is complex to make and, therefore, relatively expensive. There is a further problem associated with anodes of this type in that even if the sheets facing the diaphragm, ie the working sheets, are made continuous without a gap there is still a problem of the connecting spring between the central conductor and the working faces. From the point of view of resilience it is desired to use thin spring members. However, these have a low conductivity and can cause overheating problems. If the spring members are thickened to increase the electrical conductivity they become less effective as spring members.
By film-forming or valve metal as used herein is meant a metal chosen from the group titanium, zirconium, niobium, hafnium or tantalum or an alloy of one or more of these metals having comparable anodic properties. By anodically active material is meant a material capable of operating as an anode, of passing an electrical current without passivating and without rapidly dissolving.